prefer-type-guard.md

prefer-type-guard

A common idiom in JavaScript to differentiate between two possible types is to check for the presence in the object of a member of the desired type. Usually, to simplify the code, a boolean function is created to check the type.

Typescript provides user defined type guard functions. These are just functions with a return type of argumentName is SomeType. Such functions return true if the argument is of the specified type. One of the advantages of using such a function is that in a conditional block where the condition is a type guard, the compiler automatically performs the appropriate casts, so explicit casting becomes unnecessary.

This rule raises an issue when a boolean function checking for the type of its only argument can be replaced with a user-defined type guard function.

Noncompliant Code Example

function isSomething(x: BaseType) : boolean { // Noncompliant
  return (<Something>x).foo !== undefined;
}

if (isSomething(v)) {
  (<Something>v).foo();
}

Compliant Solution

function isSomething(x: BaseType) : x is Something {
  return (<Something>x).foo !== undefined;
}

if (isSomething(v)) {
  v.foo();
}

See

TypeScript advanced types